Dual sim dual standby (DSDS) devices are designed to share a single radio frequency resource (referred to hereinafter interchangeably as the RF or the RF resource(s)) between two subscriber identity modules (SIMs). Each SIM is associated with a subscription and a protocol stack. So, at any point of time, the RF is dedicated to only one SIM. The RF may not be associated with an intended subscription when a network tries to page the DSDS device due to mobile termination (MT) call, SMS, etc. Even if the RF is acquired after the activity of the other subscription, it still introduces delay in receiving a page message or sometimes not receiving a page message as the network will retransmit the page message. Sometimes the network may terminate the call establishment procedure if the DSDS device does not respond to the page message within a specified time. This may contribute to degradation of the user experience of the caller when an MT call is made to the intended subscription. Since the DSDS device may be in any state when paging is expected, different mechanisms are proposed to overcome specific scenarios.
For instance, consider a scenario, wherein packet-switch (PS) session is established in the first SIM (SIM1) (CELL_DCH) and the second SIM (SIM2) is in idle mode. During the ongoing PS session of SIM1, RF activity in SIM1 may be interrupted due to paging, measurements, cell search or system information block (SIB) reading on SIM2. When the RF is tuned to SIM2, the probability of missing packet data units (PDU's) of a signaling message on SIM1 is relatively high. This situation may cause the first subscription to miss PDU sequence numbers (SN's) and may render the layer-2 recovery procedure ineffective. As part of the layer-2 recovery procedure, status reports are sent to the network to indicate missed SN's for re-transmission. However, if the RF is not tuned to SIM1, re-transmissions may not be received which may stall reception. Even in a case where paging type 2 PDUs are received at the DSDS device, layer-2 (L2) will not forward it to radio resource control (RRC) as in most cases in-sequence delivery will be configured as true.
FIG. 1 is a signal flow diagram 100 illustrating an example problem scenario in which a UE is unable to receive a packet data unit due to RF tune away. According to FIG. 1, the signal flow diagram 100 illustrates the interaction between user equipment (UE) 102, an access network 104, and a core network 106, wherein the UE is a DSDS device. According to the flow chart 100, initially, the access network 104 continuously transmits packet data units (PDUs) to the UE 102 along with sequence numbers (SN's), such as SN0, SN1, SN2, and the like. During transmission, as the UE 102 is a DSDS device, periodically, the radio frequency (RF) resource gets tuned away (depicted as RF Tune Away 1) and the receiving of SN2 gets obstructed. As UE 102 receives SN3 after returning back of the RF resource, it misses SN2 and thus transmits a status PDU message to the access network 104 asking about the status of SN2 (depicted as Tx 1).
Upon receiving the status PDU for SN2, the access network 104 retransmits SN2 to the UE 102. But again the RF resources get tuned away (depicted as RF Tune Away 2) and thus receiving of SN2 fails again. The core network 106 transmits an MT paging message to the access network 104. The process of not receiving SN2 continues for N number of RF tune away sessions. Further, the UE 102 after a determined time period transmits a status PDU message to the access network 104 (depicted as Tx N). The access network 104 transmits SN2 to the UE 102, and also transmits a paging type2 PDU message to the UE 102.
Consider another scenario, in which a circuit-switch/packet-switch (CS/PS) session is established in the third generation of wireless mobile telecommunications technology (3G), various parameters and average signal-to-interference ratio (SIR) and block error rate (BLER) performances are evaluated to determine an “Out of Sync” condition by the physical layer. If the average SIR becomes less than threshold, the physical layer starts reporting Out of Sync to layer 3. When N313 consecutive Out of Sync indications are reported to layer 3, a T313 timer (Typical value of T313 timer is 4-5 seconds) is started. After T313 expiry criteria for radio link failure is fulfilled, a timer T314/T315 is started. If the UE finds a cell before T314/T315 expiry, a cell update procedure is triggered.
FIG. 2 is a signal flow diagram 200 illustrating an example problem scenario in which the UE is unable to transmit a status packet data unit to a network. In some example embodiments, the signal flow diagram 200 includes a similar or the same description as the above-described example embodiments in association with FIG. 1. Redundant descriptions between FIGS. 1 and 2 may be omitted. According to FIG. 2, the signal flow diagram 200 illustrates the interaction between user equipment (UE) 202, an access network 204, and a core network 206, wherein the UE 202 is a DSDS device. According to the flow chart 200, initially, the access network 204 continuously transmits packet data units (PDUs) to the UE 202 along with sequence numbers (SN's), such as SN0, SN1, SN2, and the like. During transmission, as the UE 202 is a DSDS device, periodically, the radio frequency (RF) resource gets tuned away and the receiving of SN2 gets obstructed. As UE 202 receives SN3 after returning back of the RF resource, it misses SN2 and thus transmits a status PDU message to the access network 204 asking about the status of SN2.
After poll timer expiry, the access network 204 retransmits SN2 to the UE 202 with the pollbit set (depicted as Re-Tx1 and SN3 PollBit=1). But again the RF resource gets tuned away and thus receiving of SN2 fails again. While UE 202 is attempting to transmit the status PDU request again, the access network 204 experiences loss of signal in uplink when the UE 202 gives away RF to the protocol stack of SIM2 (stack2) while the protocol stack of SIM1 (stack1) is connected to the access network 204, when there is excessive interference in uplink frequency or when the UE 202 is far away from base station (depicted as Tx 1). In most of the situations, the access network 204 would be able to regain synchronization with the UE 202. However, there is a chance that the access network 204 would not be able to receive the signal from the UE 202, for example, the UE's transmission may not reach the access network 204.
In such cases, if there is pending data not yet acknowledged by the UE 202, the access network 204 may keep re-sending the same PDU after poll timer expiry with the poll bit set (depicted as Re-Tx2, Re-Tx3, and Re-TxN) and the UE 202 receives the poll bit set (depicted as Re-Rx1, Re-Rx2, and Re-Rxi). The UE 202 sends a status PDU to inform the access network 204 about the PDU SN2 that has not been received (depicted as Txj, j<N). If this status PDU is not received at the access network 204 due to reasons described above, the access network 204 would stop DPCH transmission after several retransmissions of same PDU during its waiting time period (depicted as NW stops Tx(DPCH)). When the SIR measured at the UE 202 drops, a timer T313 is started as defined by the 3rd Generation Partnership Project (3GPP). This timer value is generally 3˜4 seconds during which the UE 202 continues to monitor the SIR. In most occasions, the T313 timer tends to expire before radio link control (RLC) retransmissions exceed an upper limit and hence RLC recovery procedure is not considered here. Upon T313 timer expiry, the UE 202 releases dedicated channel (DCH) resources and performs cell search procedure to camp and send a Cell Update to the access network 204. If there is any pending paging (e.g., the depicted MT Paging) message from the access network 204 to be sent to the UE 204, it will be delayed until a cell update procedure is performed or a RRC connection release waiting time is reached, whichever is earlier.
FIG. 3 is a signal flow diagram 300 illustrating an example problem scenario in which the UE is unable to transmit a pending data packet data unit to a network. In some example embodiments, the signal flow diagram 300 includes a similar or the same description as the above-described example embodiments in association with FIGS. 1 and 2. Redundant descriptions between FIGS. 1 and 2, and FIG. 3 may be omitted. According to FIG. 3, the signal flow diagram 300 illustrates the interaction between user equipment (UE) 302, an access network 304, and a core network 306, wherein the UE 302 is a DSDS device. According to the flow chart 300, initially, the access network 304 continuously transmits packet data units (PDUs) to the UE 302 along with sequence numbers (SN's), such as SN0, SN1, SN2, and the like. During transmission, as the UE 302 is a DSDS device, periodically, the radio frequency (RF) resource gets tuned away (depicted as RF Tune Away 1, RF Tune Away 2, RF Tune Away N+1, and RF Tune Away N+i) and the receiving of SN2 gets obstructed. As UE 302 receives SN3 after returning back of the RF resource, it misses SN2 and thus transmits a status PDU message to the access network 304 asking about the status of SN2.
After poll timer expiry, the access network 304 retransmits the SN2 to the UE 302 with the pollbit set. But again the RF resource gets tuned away and thus receiving of SN2 fails again. While UE 302 is attempting to transmit the status PDU request again, the access network 304 experiences loss of signal in uplink when the UE 302 gives away RF to stack2 while stack1 is connected to the access network 304, when there is excessive interference in uplink frequency or when the UE 302 is far away from base station. In most of the situations, the access network 304 would be able to regain synchronization with the UE 302. However, there is a chance that the access network 304 would not be able to receive the signal from the UE 302, for example, the UE s transmission may not reach the access network 304.
In such cases, the UE 302 would keep re-sending the pending data PDU after poll timer expiry if L2 acknowledgement is not received from the access network 304 (depicted as Tx1, TxN, Re-Tx1, Re-Tx2 and Re-Txi; the re-sent signal is depicted as UL SN X). If synchronization between the UE 302 and the access network 304 is not restored and the access network 304 not able to receive data in uplink (UL) due to any of the reasons described above, the access network 304 would stop dedicated physical channel (DPCH) transmission after its waiting time period.
When the SIR measured at the UE 302 drops as DPCH transmission is stopped from the access network 304, a timer T313 is started as defined by 3GPP. The duration of this timer is generally 3˜4 seconds during which the UE 302 continuously monitors the SIR. A SIR drop at the UE 302 may happen for various reasons, including when the access network 304 stops DPCH transmission. In most occasions, the T313 timer tends to expire before RLC retransmissions exceed the upper limit and hence RLC recovery procedure is not considered here. Upon T313 timer expiry, the UE 302 releases DCH resources and performs cell search procedure to camp and send a Cell Update to the access network 304. If there is any pending paging message from the access network 304 to be sent to the UE 302, it will be delayed until a cell update procedure is performed or a RRC connection release waiting time is reached, whichever is earlier.
Due to multiple instances of RF tune away to other stack, the network may lose uplink synchronization with the DSDS device as the DSDS device's transmitter (TX) is operating in discrete mode. Due to this condition, the access network may turn off its transmission unless uplink synchronization or good cyclic redundancy check (CRC) PDU's are received in uplink. The DSDS device will start a recovery procedure of sending a cell update only after the timer expiry as mentioned in above section. However, the network may release the RRC connection before the DSDS device attempts to re-establish the connection.
In view of the foregoing, methods for improving Paging Type2 performance in Dual Sim Dual Standby (DSDS) device would be desirable.